Laser processing of dielectric and conductive materials is commonly used to ablate fine features in electronic components. For example, chip packaging substrates may be laser processed in order to route signals from the semiconductor die to a ball-grid array or similar package. Laser processed features may include signal traces, ground traces, and microvias (to connect signal traces between package layers). A recent design trend incorporates signal and ground traces on a single layer to tightly control signal impedance while reducing the number of layers in a chip package. Such an approach may require small feature dimensions and spacing (e.g., about 10 microns (μm) to about 25 μm), and long trace lengths per package (e.g., about 5 meters (m) to about 10 m). In order to construct chip packages economically, the speed at which such features are ablated may be quite high (e.g., from about 1 meter/second (m/s) to about 10 m/s). Certain packages may be processed, for example, in about 0.5 second (s) to about 5 s to meet customer throughput goals.
Another useful characteristic of chip packaging may be to provide intersecting traces with controlled depth variation. For example, ground traces may branch at several points throughout the pattern. At each branching intersection, the traces may be ablated with a desired depth variation of less than about +/−10%. Normally, if two trenches were to be ablated at one point, the double exposure of the ablating beam would create a depth variation of about 100%.
Another useful characteristic of chip packaging may be to provide variable trace widths at different parts of the package to control impedance or provide pads for inter-layer connection vias. Trace width control should be provided with reduced or minimal disruption to the high-velocity processing of the main trace.
It may also be useful to process features of arbitrary size and shape, at high speed, with reduced or minimal time used to change the feature's characteristics. For example, features may include microvias with a variety of diameters and/or sidewall taper, square or rectangular pads, alignment fiducials, and/or alphanumeric notation. Traditionally, for processing features such as microvias, optical systems have been designed to provide shaped intensity profiles (e.g., flat-top beams) of variable diameter, or purely Gaussian beams. These optical systems may have significant time delays (e.g., about 10 milliseconds (ms) to about 10 s) when changing laser processing spot characteristics.
Other problems are associated with building a machine to meet the processing parameters noted above. For example, traces may change direction throughout the package due to routing requirements. When processing traces at high velocity, the variation in trajectory angle may require high beam position acceleration at very short time scales. Laser processing can easily exceed the dynamic limits of the beam positioner, for example, when running at the high velocities (e.g., about 1 m/s to about 10 m/s) used for high throughput.
Such accelerations and/or velocities may be difficult to achieve in traditional laser processing machines, which have relied on beam positioning technologies such as linear stages in combination with mirror galvanometer beam deflectors (referred to herein as “galvos” or “galvo mirrors”), along with static (or slowly varying) beam conditioning optics that cannot respond in the time scales used for this type of processing (e.g., on the order of about 1 microsecond (μs) to about 100 μs).
The actual ablation process may also be a factor to consider. Laser pulses with high peak power may be used to ablate the dielectric material while minimizing thermal side effects such as melting, cracking, and substrate damage. For example, ultrafast lasers with pulse widths in a range between about 20 picoseconds (ps) and about 50 ps at repetition rates of about 5 megaHertz (MHz) to about 100 MHz can process materials with high peak power while providing significant pulse overlap to avoid pulse spacing effects. Fiber lasers now commonly provide pulse widths in the nanosecond region at repetition rates of greater than about 500 kiloHertz (kHz). Normally, for a given process condition (ablation depth and width), the “dosage” (power/velocity) applied to the processed material should be constant. However, at low velocities, the applied power may become so low that the peak pulse power may be insufficient to ablate the material without inducing thermal effects (e.g., melting and charring).
Another processing effect that can reduce ablation efficiency may be the interaction of the processing beam with the plume of ablated material. Plumes may distort or deflect the beam enough to disrupt the focused beam, or cause accuracy issues due to its deflection.
Beam positioner designs may deflect the process beam using galvos. The intensity profile of the process beam at a workpiece may be Gaussian (for simple focusing of a Gaussian beam), or a shaped intensity profile (e.g., flat-top profile) for beams conditioned by a fixed optic beam shaper.
Systems have been described in which acousto-optic deflectors (AODs) have been combined with galvos to provide high-speed deflection, for example in U.S. Pat. Nos. 5,837,962 and 7,133,187. However, these references do not describe obtaining the desired performance in advanced beam positioner designs.